Most photometric analyses are made using a cell or probe to isolate a sample to be analyzed from a light source and a photodetector. In the case of a cell, the cell body contains a cavity in which the sample is placed and the body comprises two windows so that light from the source can be passed through one of the windows, through the sample, through the other window and then to the photodetector. The cell can be made entirely of a material transparent to the light, e.g., a test tube, or can be made with separate windows mounted and sealed to a cell body and even separate windows mounted and sealed to a metallic body. One advantage of a cell having separate windows and a metallic body is that it can be built to withstand higher sample temperature and pressure, e.g., a cell connected to a high pressure and/or high temperature chemical process so that a fluid sample stream from the process can be passed through the cavity of the cell and photometrically analyzed, e.g., the high pressure infrared cell of Walker et al. U.S. Pat. No. 3,886,364.
Conventional cells of the Walker et al. type employ a gasket material between the windows and the metallic cell body (or between the windows and removable metallic window mounts which are removable from the cell body to facilitate replacement of the windows). Many advancements have been made in gasket sealing such windows to reduce the problem of sample leaking between the windows and the metallic cell body (or between the windows and the metallic window mounts) for high temperature and/or high pressure applications. See, for example, Papanek et al., The Review of Scientific Instruments, May, 1972, page 738-739: Tinker et al., The Review of Scientific Instruments, July, 1972, pages 1024-1026; Noack, Spectrochimica Acta, Vol. 24A, 1968, pages 1917-1920: Penninger, Journal of Catalysis, 56, 1979, pages 287-289; and Fong et al., Applied Spectroscopy, Vol. 24, Number 1, 1970, pages 21-27. One problem the conventional cells still have is a tendency to leak after extended usage, e.g., as the gaskets deteriorate. This can cause the windows to fog with leaked sample on the side of the window opposite the sample side which can interfere with the accuracy of analysis. Worse yet, some leaks can result in damage to other parts of the photometric analysis system and if the leak is severe can even cause an industrial accident.
In the case of a probe used in photometric analysis, a window is generally mounted and sealed at an end of a metallic hollow tube-shaped body. The window end of the probe can be immersed into a sample while light is directed through the window into the sample and then, sometimes after bouncing off a mirror and sometimes after bouncing off a particle in the sample, back through the window to a photodetector. The light is often directed from the source to the probe and then from the probe to the photodetector by optical fibers. The window can be a prism for Attenuated Total Reflectance (ATR) analysis of the sample. The probe can be mounted in a high pressure and/or high temperature chemical process line or reactor, e.g., the fiber optic probe of McLachlan et al U.S. Pat. No. 4,707,134.
Conventional probes of the McLachlan et al. type employ a gasket material between the window and the metallic probe body. One problem of these conventional probes is a tendency to leak after extended usage in high temperature and/or high pressure industrial applications, e.g., as the gaskets deteriorate, with the same deleterious results as noted above with gasket sealed windows in photometric cells.
Sealing a window to metallic bodies using a brazed seal is known. Chan et al. U.S. Pat. No. 4,179,037 describes a xenon arc lamp having a sapphire window, a Kovar alloy body and a brazed seal between the window and the body. Rockwell U.S. Pat. No. 3,555,450 describes a laser having a quartz window, a metal window mount and a brazed seal between the window and the window mount. U.S. Pat. Nos. 3,281,174, 3,289,291, 3,323,821, 3,415,556, 3,736,650, 3,281,523 and 3,969,126 (all assigned to Varian Associates of Palo Alto, Calif.) teach many aspects of brazed seals. The aforementioned publications and patents are fully incorporated herein by reference. The Eimac Division of Varian Associates sells sapphire and other windows brazed to metallic bodies including sapphire windows brazed near the end of a short tube.
Sealing a window to bodies using a frit seal is known. Liberman et al. U.S. Pat. No. 4,666,251 describes a metal vapor Raman cell having very large windows using a frit seal. Wright et al. U.S. Pat. No. 4,063,803 describes a laser having a window sealed to the laser tube using a frit seal. Sinclair Manufacturing, Inc. of Chartley, Mass., sells sapphire and other windows frit sealed to metal bodies. Fusion sealing windows to ceramic cell bodies (such as quartz windows to quartz cell bodies) is well known for atmospheric pressure photometric cells.